As preparation process for glyoxal, there have generally been known processes relying upon the oxidation of acetylene or ethylene, the oxidation of acetaldehyde with nitric acid, the oxidation of ethylene glycol, and the like. However, the process relying upon the oxidation of acetaldehyde with nitric acid is mainly employed in the industry.
The oxidation of acetaldehyde with nitric acid requires, as an oxidizing agent, nitric acid in an amount at least equal by mole to the acetaldehyde to be reacted. It is thus accompanied by such shortcomings that unreacted nitric acid is unavoidably mixed and organic acids are by-produced as impurities in relatively large amounts, thereby making a complicated separation and purification step indispensable.
On the other hand, a number of proposals have been made for the process for preparing glyoxal by oxidizing ethylene glycol. There are, for example, a process for effecting the oxidation with oxygen by using an oxidizing catalyst which is made of copper and/or silver and phosphorus (Japanese Patent Publication No. 1364/1973) and an oxidation process which is carried out, in the presence of a catalyst containing phosphorus and copper, phosphorus and silver, or phosphorus, copper and silver, by incorporating a bromine compound within an amount which does not lower the conversion of ethylene glycol to about 90% or less in the mixed feed gas (Japanese Patent Laid-open No. 17408/1977). It has also been proposed to use a copper-containing catalyst (U.S. Pat. No. 2,339,282) and to effect the oxidation in the simultaneous presence of a copper-containing catalyst and a halogen compound in a cylindrical reactor made of a Cu-Si-Mn alloy. However, the above processes which employ these alloy-based catalysts were not significantly advantageous from the industrial viewpoint, since the preparations of the catalysts were difficult, their service life in which they can maintain reaction results at high levels was short, and the regeneration treatments of the catalysts were complicated.
As processes relying upon the oxidation of ethylene glycol, it has also been proposed to effect the oxidation in the presence of silver crystals
having uniform particle sizes (0.1-2.5 mm)--Japanese Patent Laid-open No. 103809/1979--and to carry out the oxidation by using a copper-containing catalyst and in the simultaneous presence of a phosphorus compound which is vaporized under reaction conditions (Japanese Patent Laid-open No. 55129/1980). The yields of glyoxal by these processes were not significantly high and these processes are not satisfactory as industrial preparation processes.
However, the preparation process for glyoxal relying upon the gas phase oxidation of ethylene glycol has by itself been considered to be an advantageous process from the standpoint of economy because it uses, as a raw material source, ethylene glycol which is a derivative of ethylene oxide obtained from inexpensive natural gas as its starting raw material and is thus economically superior compared with the oxidation of acetaldehyde with nitric acid.